The present invention relates to an apparatus and method for threading the end of the wire electrode of a travelling wire EDM apparatus through a pre-drilled aperture in the workpiece, or through the cut previously machined by electrical discharges in the workpiece, in the event of rethreading the electrode wire after rupture. More specifically, the present invention relates to a method and mechanism for threading the end of the electrode wire through the workpiece by entraining and centering the end of the electrode wire by a fluid jetstream directed towards the aperture in the workpiece.
It is known, in travelling wire EDM technology, to utilize a fluid jetstream for threading an electrode wire through an aperture in a workpiece. In U.S. Pat. Nos. 3,891,819 and 3,987,270 and in Japanese patent publication No. 56-10130, there are disclosed electrode wire threading devices comprising a nozzle through which the end of the electrode wire is fed longitudinally by a mechanical wire feeding device, the nozzle being directed towards the threading aperture in the workpiece and being supplied with a fluid under pressure such as to cause the fluid to flow in the form of a jetstream directed towards the threading aperture and coaxially surrounding the electrode wire. The end of the electrode wire is guided by the fluid jetstream and is, at least partially, driven towards the threading aperture in the workpiece as a result of the skin friction effect between the jetstream of fluid and the peripheral surface of the electrode wire.
In such electrode wire threading arrangements, the threading aperture through the workpiece does not play any active part. On the contrary, the threading aperture itself presents an obstacle to effective threading of the electrode wire end, especially when the threading aperture is narrower than the jetstream provided by the nozzle located proximate the inlet of the threading aperture. This is due to the fact that the fluid jetstream becomes partially broken and disturbed at the edge of the threading aperture, and the fluid jetstream jacketing the electrode wire effectively guides and drives the wire only in the short free space between the nozzle outlet and the inlet of the threading aperture through the workpiece. Under such conditions, it is evident that if the longitudinal axis of the electrode wire does not coincide exactly with the longitudinal axis of the threading aperture, chances are that the end face of the wire impinges on the edge of the aperture and the wire is caused to bend, proximate its end, under the action of the force exerted on the wire by the mechanical wire feeding device. It has been found experimentally that, because of the inconveniences hereinbefore mentioned of the prior art threading mechanisms, such threading mechanisms do not permit to thread, or re-thread, an electrode wire of very small size, as small as, for example 0.1 mm, through a threading aperture two or three times the size of the wire, such as, for example, 0.2 to 0.3 mm in diameter. The end of the wire is often not disposed concentric to the jetstream and consequently not concentric to the threading aperture because the wire is generally lacking in straightness because, for example, it is supplied from a relatively small diameter spool, or has been subjected to stress.
It has been suggested to straighten the electrode wire, prior to threading it through a threading aperture in a workpiece, by heating the wire while simultaneously stretching the wire. This process is particularly convenient for straightening wires made of copper and of alloys containing a high proportion of copper. Copper melts at about 1,000.degree. C. and, when subjected to low stress, it is capable of drastic deformation at a temperature of the order of 600.degree. C. By contrast, wires made of tungsten and/or molybdenum have a melting temperature of about 3,000.degree. C., and it is very difficult, if not impossible, to subject such wires to an appropriate thermal treatment.
It is evident that any automatic wire threading mechanism, even if far from being perfect, can accomplish satisfactory threading operations on the condition that the size of the threading aperture through the workpiece is several order of magnitudes, for example five times, that of the wire. However, pre-drilling an aperture of such a size through the workpiece requires time and, often, it is not possible to pre-drill an aperture of large size because the cut in the workpiece does not permit to accommodate a starting aperture of such large dimension. The modern trend to miniaturization often requires that stamping or extrusion dies be made of elements having very small lateral dimensions, for example, from 0.1 to 0.2 mm. In order not to run the risk of irremediably damaging the workpiece and being forced to scrap it, the threading aperture through the workpiece must be of a size which can be accommodated within the largest dimension of the opening within the confines of the die walls. Therefore, there is often a requirement for forming a threading orifice through a workpiece of a size which is hardly larger than the size of the electrode wire, with the accompanying difficulties of threading the electrode wire through such a small size aperture presenting a problem which cannot be resolved by the prior art threading mechanisms and methods.